Mismatch Detected for 'RuntimeLibrary'

I downloaded and extracted Crypto++ in C:\cryptopp. I used Visual Studio Express 2012 to build all the projects inside (as instructed in readme), and everything was built successfully. Then I made a test project in some other folder and added cryptolib as a dependency. After that, I added the include path so I can easily include all the headers. When I tried to compile, I got an error about unresolved symbols.

(This is already answered in comments, but since it lacks an actual answer, I'm writing this.)

This problem arises in newer versions of Visual C++ (the older versions usually just silently linked the program and it would crash and burn at run time.) It means that some of the libraries you are linking with your program (or even some of the source files inside your program itself) are using different versions of the CRT (the C RunTime library.)

To correct this error, you need to go into your Project Properties (and/or those of the libraries you are using,) then into C/C++, then Code Generation, and check the value of Runtime Library; this should be exactly the same for all the files and libraries you are linking together. (The rules are a little more relaxed for linking with DLLs, but I'm not going to go into the "why" and into more details here.)

There are currently four options for this setting:

Multithreaded Debug

Multithreaded Debug DLL

Multithreaded Release

Multithreaded Release DLL

Your particular problem seems to stem from you linking a library built with "Multithreaded Debug" (i.e. static multithreaded debug CRT) against a program that is being built using the "Multithreaded Debug DLL" setting (i.e. dynamic multithreaded debug CRT.) You should change this setting either in the library, or in your program. For now, I suggest changing this in your program.

Note that since Visual Studio projects use different sets of project settings for debug and release builds (and 32/64-bit builds) you should make sure the settings match in all of these project configurations.

For (some) more information, you can see these (linked from a comment above):

UPDATE: (This is in response to a comment that asks for the reason that this much care must be taken.)

If two pieces of code that we are linking together are themselves linking against and using the standard library, then the standard library must be the same for both of them, unless great care is taken about how our two code pieces interact and pass around data. Generally, I would say that for almost all situations just use the exact same version of the standard library runtime (regarding debug/release, threads, and obviously the version of Visual C++, among other things like iterator debugging, etc.)

The most important part of the problem is this: having the same idea about the size of objects on either side of a function call.

Consider for example that the above two pieces of code are called A and B. A is compiled against one version of the standard library, and B against another. In A's view, some random object that a standard function returns to it (e.g. a block of memory or an iterator or a FILE object or whatever) has some specific size and layout (remember that structure layout is determined and fixed at compile time in C/C++.) For any of several reasons, B's idea of the size/layout of the same objects is different (it can be because of additional debug information, natural evolution of data structures over time, etc.)

Now, if A calls the standard library and gets an object back, then passes that object to B, and B touches that object in any way, chances are that B will mess that object up (e.g. write the wrong field, or past the end of it, etc.)

The above isn't the only kind of problems that can happen. Internal global or static objects in the standard library can cause problems too. And there are more obscure classes of problems as well.

All this gets weirder in some aspects when using DLLs (dynamic runtime library) instead of libs (static runtime library.)

This situation can apply to any library used by two pieces of code that work together, but the standard library gets used by most (if not almost all) programs, and that increases the chances of clash.

What I've described is obviously a watered down and simplified version of the actual mess that awaits you if you mix library versions. I hope that it gives you an idea of why you shouldn't do it!